Rotor blade for helicopter

ABSTRACT

A rotor blade for a helicopter having: a main blade with a blade root attached to a rotor shaft of the helicopter; and a small blade attached to a blade tip part of the main blade, wherein a tip part of the small blade is rectangular form in its plane shape, and has a leading edge continuous with a leading edge of the main blade and a chord length shorter than a chord length of the main blade.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Applications No. 2004-159525 filed on May 28, 2004 respectively which are incorporated herein by reference in its entirety.

BACKGROUD OF THE INVENTION

1. Field of the Invention

The present invention relates to a rotor blade for a helicopter.

2. Description of the Related Art

Helicopters have been used in various fields, such as transportation of supplies, lifesaving, and national defenses. When a helicopter lands, as shown in FIG. 13, a succeeding rotor blade 100 b interacts with tip vortices 101 generated by the blade tip of a preceding rotor blade 100 a, and generates noise. This noise is called BVI (blade vortex interaction) noise.

In conventional rotor blades, there have been used, for example, a rectangular blade tip part 200 shown in FIG. 14, and a blade tip part 300 shown in FIG. 15 that has a shape of gradually sweeping back toward the outer side of the blade tip part. Employment of rotor blades having these shapes of blade tip parts has not been able to reduce the BVI noise, though. For solving this problem, various technologies have been proposed for reducing the BVI noise, each technology adopting a rotor blade that has a specific shape (structure) of blade tip part to weaken the tip vortices.

For instance, as shown in FIG. 16, such technology has been proposed that a rectangular small blade 410 is attached to a rectangular blade tip part 400 of a rotor blade at the leading edge side to weaken tip vortices with the tip vortices generated by the rotor blade divided into two parts. As shown in FIG. 17, a blade tip part 500 of a rotor blade is provided with a tip blade 510 attached thereto having a mean chord length and a span length longer than 50% of the center-portion chord length of the rotor blade to generate two tip vortices of substantially the same intensity (refer to JP-Tokukaihei-4-262994A).

As another example, as shown in FIG. 18, a rotor blade (refer to JP-Tokukai-2002-284099A) consists of a main blade 600 and a small blade 700 that has a chord length and a span length set in a certain range. Each of a blade tip part 610 of the main blade 600 and a blade tip part 710 of the small blade 700 has a curved shape (for example, a parabolic shape). With this structure, tip vortices shed from the tip part 610 interferes with tip vortices shed from the tip part 710, and this interference can reduce the vortices.

In the technology disclosed in JP-Tokukaihei-4-262994A, however, generated two vortices are not positively interfered with each other, and therefore BVI noise could not be greatly reduced. In the technology disclosed in JP-Tokukai-2002-284099A, although generated two vortices can be interfered with each other, a problem arises in that designing and manufacturing of the small blade require substantial cost and time because the blade tip part of the small blade has a complicated shape (for example, a parabolic shape).

SUMMARY OF THE INVENTION

An object of the invention is to greatly reduce the cost and time necessary for designing and manufacturing a rotor blade of a helicopter, and to remarkably reduce BVI noise generated when the helicopter lands or descends.

For solving the problems, in accordance with the first aspect of the present invention, the rotor blade for a helicopter comprises:

-   -   a main blade with a blade root attached to a rotor shaft of the         helicopter; and     -   a small blade attached to a blade tip part of the main blade,     -   wherein the small blade is rectangular in its plane shape, and         has a leading edge continuous with a leading edge of the main         blade and a chord length shorter than a chord length of the main         blade.

According to this structure, since the small blade is attached and extended along the leading edge of the blade tip part of the main blade, tip vortices can be shed from both the tip part of the main blade and the blade tip part of the small blade. By properly setting a positional relationship between the tip part of the main blade and the blade tip part of the small blade, the tip vortices shed from the tip part of the main blade and that shed from the blade tip part of the small blade can be positively interfered with each other and diffused. As a result, the tip vortices shed from the blade tip part of the rotor blade can be weakened, which can remarkably reduce BVI noise when the helicopter lands or descends.

Further, since the small blade has a simple plane shape (rectangular shape), the time and cost required for designing and manufacturing the small blade can be greatly reduced. Further, since the rotor blade according to the invention can be manufactured by modifying a conventional rotor blade at its blade tip part only, it is possible to reduce the time and cost required for manufacturing a whole rotor blade.

Preferably, the chord length c1 of the small blade is set so as to satisfy a following relational expression; α≦c1≦0.5C (α>0) where C denotes the chord length of the main blade.

According to this structure, since the chord length of the small blade is set to a certain length (50% or less of the chord length of the main blade), the intensity of the tip vortices (swirl velocity) shed from the blade tip part of the main blade and that shed from the blade tip part of the small blade can be properly set. Resultantly, these two tip vortices can be effectively interfered with each other, thereby achieving superior effect of BVI noise reduction.

Preferably, the chord length c1 of the small blade is so set as to satisfy a following relational expression; 0.2C≦c1≦0.5C where C denotes the chord length of the main blade.

With this relationship, since the chord length of the small blade is set to a certain length (20% or more and 50% or less of the chord length of the main blade), the intensity of the tip vortices (swirl velocity) shed from the blade tip part of the main blade and that shed from the blade tip part of the small blade can be more properly set. Resultantly, these two tip vortices can be more effectively interfered with each other, thereby achieving superior effect of BVI noise reduction.

Preferably, a blade tip part of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is so set as to satisfy a following expression; β≦b1≦0.5C (β>0) where C denotes the chord length of the main blade.

According to this structure, since the blade tip of the small blade protrudes outward by a specific length (50% or less of the chord length of the main blade) relative to the tip part of the main blade, the vortex center position of the tip vortices shed from the blade tip part of the main blade can be separated apart from that shed from the blade tip part of the small blade in the span direction. As a result, these two tip vortices can be more positively interfered with each other, leading to superior effect of BVI noise reduction.

Preferably, a blade tip part of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is so set as to satisfy a following expression; 0.2C≦b1≦0.4C where C denotes the chord length of the main blade.

With this relationship, since the blade tip of the small blade protrudes outward by a specific length (20% or more and 40% or less of the chord length of the main blade) relative to the tip part of the main blade, the vortex center position of the tip vortices shed from the blade tip part of the main blade can be separated apart from that shed from the blade tip part of the small blade by a proper distance in the span direction. As a result, these two tip vortices can be more positively interfered with each other, thereby achieving superior effect of BVI noise reduction.

Preferably, a blade tip vicinity part of the main blade is bent downward by a predetermined anhedral angle.

According to this structure, since the blade tip vicinity part of the main blade is bent downward by a predetermined anhedral angle, if the small blade does not have a predetermined anhedral angle relative to the main blade, tip vortices generated by the blade tip part of the main blade can be positioned under tip vortices generated by the blade tip part of the small blade. Therefore, these two tip vortices can be effectively interfered with each other. Further, the tip vortices are shed downward from the preceding rotor blade and hard to interact with the succeeding rotor blade, which can reduce BVI noise all the more.

Additionally, since the preceding rotor blade can shed tip vortices downward, the partial stall of the succeeding rotor blade becomes smaller, the stall generally being caused by the current induced by the tip vortices. Resultantly, there can be reduced energy loss during the drive of a rotary wing, and be improved hovering performance while the helicopter is suspended in the air.

Preferably, the small blade has a predetermined anhedral angle relative to the main blade.

According to this structure, since the small blade is bent downward by a predetermined anhedral angle relative to the main blade, the tip vortices generated by the blade tip part of the small blade can be positioned under the tip vortices generated by the blade tip part of the main blade. Therefore, these two tip vortices can be effectively interfered with each other. Further, the tip vortices are shed downward from the preceding rotor blade and hard to interact with the succeeding rotor blade, which can reduce BVI noise all the more.

Additionally, since the preceding rotor blade can shed tip vortices downward, the partial stall of the succeeding rotor blade becomes smaller, the stall generally being caused by the current induced by the tip vortices. Resultantly, there can be reduced energy loss during the drive of a rotary wing, and be improved hovering performance while the helicopter is suspended in the air.

Preferably, the small blade has a predetermined angle of incidence relative to the main blade.

According to this structure, setting the angle of incidence to a proper value relative to the main blade allows the tip vortices shed from the small blade tip part to be adjusted in the flowing direction and density thereof. Therefore, the tip vortices shed from the main blade tip part can effectively interfere with the tip vortices shed from the small blade tip part.

Preferably, a blade tip vicinity part of the main blade and the small blade continuously joined to the blade tip vicinity part are swept back toward the outside of the blade tip.

With this structure, since the tip vicinity part of the main blade and the small blade continuously joined to the tip vicinity part sweep back toward the tip outside, there can be reduced airspeed in the direction perpendicular to the blade tip vicinity part of the main blade and the extended direction of the small blade. Consequently, this rotor blade can reduce the BVI noise, and also improve a transonic characteristic and delay the generation of a shock wave.

According to the invention, the small blade is attached and extended along the leading edge of the blade tip part of the main blade, and by properly setting a positional relationship between the tip part of the main blade and the blade tip of the small blade, the tip vortices shed from the blade tip part of the main blade and that shed from the blade tip part of the small blade can be positively interfered with each other and diffused. Consequently, the tip vortices shed from the blade tip part of the rotor blade can be weakened, which can remarkably reduce the BVI noise when the helicopter lands. Further, simple structure of the small blade allows remarkable reduction of time and cost required for designing and manufacturing the small blade, and resultant cost reduction for manufacturing a whole rotor blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein;

FIG. 1 is a plan view showing a blade tip vicinity part of a rotor blade according to a first embodiment of the present invention;

FIG. 2A is an explanatory diagram explaining tip vortices generated when a conventional rotor blade is used, FIG. 2B is an explanatory diagram explaining diffusion of tip vortices generated in the case that the rotor blade according to the first embodiment of the invention is used, and FIG. 2C is an explanatory diagram showing the state of the vortices shown in FIG. 2B when viewed from a back side;

FIG. 3A is a graph showing the state of generation of tip vortices in the case that the chord length of a small blade on the rotor blade is set to a length of 5% of the chord length of a main blade, FIG. 3B is a graph showing the state of generation of tip vortices in the case that the chord length of the small blade on the rotor blade is set to a length of 30% of the chord length of a main blade; and FIG. 3C is a graph showing the state of generation of tip vortices in the case that the chord length of the small blade on the rotor blade is set to a length of 50% of the chord length of a main blade;

FIG. 4 is a graph showing the relationship between the chord length of the small blade and the maximum swirl velocity of the tip vortices shed from the rotor blade;

FIG. 5A is a graph showing the state of generation of tip vortices in the case that the rotor blade does not have a small blade, FIG. 5B is a graph showing the state of generation of tip vortices in the case that the rotor blade has a small blade with a specific length set to 20% of the chord length of the main blade, and FIG. 5C is a graph showing the state of generation of tip vortices in the case that the rotor blade has a small blade with a specific length set to 40% of the chord length of the main blade;

FIG. 6 is a graph showing the relationship between the specific length and the maximum swirl velocity of the tip vortices shed from the rotor blade;

FIG. 7 is a graph showing the effect of noise reduction in the case of using the rotor blade having the main blade with the small blade;

FIG. 8A is a plan view showing a blade tip vicinity part of a rotor blade according to a second embodiment of the present invention, and FIG. 8B is a diagram showing the rotor blade of FIG. 8A when viewed from a trailing edge side;

FIG. 9A is a plan view showing a blade tip vicinity part of a rotor blade according to a third embodiment of the present invention, and FIG. 9B is a diagram showing the rotor blade of FIG. 9A when viewed from a tip side;

FIG. 10 is a plan view showing a blade tip vicinity part of a rotor blade according to a fourth embodiment of the present invention;

FIG. 11 is a plan view showing a blade tip vicinity part of a rotor blade according to a fifth embodiment of the present invention;

FIG. 12 is a plan view showing a blade tip vicinity part of a rotor blade according to a sixth embodiment of the present invention;

FIG. 13 is an explanatory view for explaining a principle of generation of BVI noise;

FIG. 14 is a plan view showing a rectangular blade tip part of a conventional rotor blade;

FIG. 15 is a plan view showing a conventional rotor blade with a blade tip part having a swept-back angle;

FIG. 16 is a perspective view explaining the construction of a blade tip vicinity part of a conventional rotor blade having a small blade;

FIG. 17 is a perspective view explaining the construction of a blade tip vicinity part of a conventional rotor blade having a small blade; and

FIG. 18 is a plan view explaining the construction of a blade tip vicinity part of a conventional rotor blade having a small blade.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

First Embodiment

Explanation will be given of the construction of a rotor blade 10 according to a first embodiment of the invention with reference to FIG. 1. A plurality of rotor blades 10 are attached to a rotor shaft 20 of a helicopter (not shown) to constitute a rotary wing. As shown in FIG. 1, the rotor blade 10 includes a main blade 11, a blade root 30 of which is attached to the rotor shaft 20, and a small blade 12 attached to the blade tip part of the main blade 11.

The main blade 11 has a substantially constant chord length C except a blade tip vicinity part, and is so formed that a trailing edge 11 b protrudes outward relative to a leading edge 11 a. The top end of the leading edge 11 a of the main blade 11 is connected to the top end of the trailing edge 11 b with a blade tip end 13 having a smoothly curved shape (parabolic shape), which constitutes a blade tip vicinity part of the main blade 11.

A tip part of the small blade 12, as shown in FIG. 1, is a rectangular form in its plane shape, and has a leading edge 12 a continuous with the leading edge 11 a of the main blade 11 and a chord length c1 shorter than the chord length C of the main blade 11. In this embodiment, the chord length c1 of the small blade 12 is so set as to satisfy a following relational expression: 0.2C≦c1≦0.5C That is, the chord length c1 of the small blade 12 is set to a length of 20% or more and 50% or less of the chord length C of the main blade 11.

The blade tip part of the small blade 12, as shown in FIG. 1, protrudes outward by a specific length b1 relative to the trailing edge end of the main blade 11. In this embodiment, the specific length b1 is so set as to satisfy a following relational expression: 0.2C≦b1≦0.4C That is, the specific length b1 is set to a length of 20% or more and 40% or less of the main blade chord length C.

Referring to FIG. 2, a description will be given of the state of tip vortex diffusion for each case of using the rotor blade 10 according to this embodiment and a conventional rotor blade 100 having a rectangular blade tip part.

In the case that the conventional rotor blade 100 is used, as shown in FIG. 2A, strong tip vortices 100 a are shed from the blade tip part, and the vortices 100 a flow backward without being diffused. On the contrary, if the rotor blade 10 of the embodiment is used, as shown in FIG. 2B, first tip vortices 11 c are shed from the blade tip part of the main blade 11, and second tip vortices 12 c from the blade tip part of the small blade 12. Each of two vortices 11 c and 12 c flows backward.

Since the blade tip of the small blade 12 protrudes outward by the specific length b1 relative to the trailing edge end of the main blade 11, the second tip vortices 12 c flow backward passing the near outside of the blade tip part of the main blade 11, and positively interfere with the first tip vortices 11 c. That is, the right side of the first tip vortices, which rotate counterclockwise 11 c in FIG. 2C, are canceled by the left side of the second tip vortices 12 c that also rotate counterclockwise, which causes the vortices to be diffused as a whole with intensity of the vortices weakened. As a result, the tip vortices shed from the rotor blade 10 are weakened, and the BVI noise is greatly reduced.

A description will be given of analysis on tip vortices carried out for setting an upper limit value and a lower limit value of the chord length c1 of the small blade 12 of the rotor blade 10 according to this embodiment, referring to FIGS. 3 and 4.

Relating to this embodiment, tip vortices generation states are analyzed in the case that a main blade of a rotor blade is provided with a small blade having various chord lengths c1. Based on the analysis result, a range of the chord length c1 (upper limit value and lower limit value) is extracted so as to obtain an expected diffusion effect on tip vortices, and applied the range of the chord length c1 to the small blade 12. Here, the main blade used in this analysis has a rectangular blade tip part.

FIGS. 3A to 3C are graphs showing tip vortices generation states in the case that the main blade of the rectangular blade tip part is provided with a small blade having the chord length c1 set to (a) 5%, (b) 30% and (3) 50% of the main blade chord length C, respectively.

The ordinate in FIG. 3 denotes values that the length, extending from the blade tip end position of the main blade in a span direction, is divided by the chord length C: zero represents the blade tip end position of the main blade, positive values the outside of the blade tip end, and negative values the blade root 30 side. The abscissa in FIG. 3 denotes the values that the length, extending in a direction of main blade thickness from the thickness center position to the upper and lower surface sides of the blade, is divided by the chord length C: zero represents the thickness center position of the main blade, positive values the upper surface side, and negative values the lower surface side. Presented in FIG. 3 are vorticity contour lines h and slip flow velocity vectors v at a downstream position with one chord length of the main blade from the main blade. The thinner vorticity contour lines h and the shorter slip flow velocity vectors v mean the better vortex diffusion.

In the case that the chord length c1 of the small blade is set to 5% of the chord length C of the main blade, as shown in FIG. 3A, the contour lines h are relatively dense and the velocity vectors v are relatively longer, therefore it is understood that tip vortices shed from the blade are relatively strong. The reason for this is: if the chord length c1 is short, while the swirl velocity of tip vortices shed from the blade tip part of the small blade is low, the swirl velocity of tip vortices shed from the blade tip part of the main blade is high, and hence these two tip vortices interfere relatively little with each other.

On the contrary, in the case that the chord length c1 of the small blade is set to 30% and 50% of the chord length C of the main blade as shown in FIGS. 3B and 3C, the contour lines h become relatively thinner and the velocity vectors v become relatively shorter, therefore it is understood that tip vortices shed from the blade are relatively weaker. The reason for this is: if the chord length c1 is set to a proper value, the lift generated by the small blade becomes large, therefore the swirl velocity of tip vortices shed from the blade tip part of the small blade is almost equal to that shed from the blade tip part of the main blade, and hence these two tip vortices effectively interfere with each other.

Incidentally, if the chord length c1 is set to more than 50% of the chord length C, then the volume and weight of the small blade become larger, and therefore the intensity of joint between the small blade and the main blade unfavorably becomes lower.

In a graph of FIG. 4, the ordinate denotes the maximum swirl velocity of tip vortices shed from the rotor blade, and the abscissa denotes values that the chord length c1 of the small blade is divided by the chord length C of the main blade. As shown by a dotted line in FIG. 4, if the main blade is not provided with a small blade, the maximum swirl velocity of tip vortices presents a constant high value. It is understood, on the contrary, that, if the main blade is provided with the small blade having the proper chord length c1 (for example, 20% to 50% of the chord length C of the main blade), the maximum swirl velocity of tip vortices is greatly reduced due to vortex diffusion of the small blade tip part.

Based on the analysis result described above relating to the rotor blade 10 according to the embodiment, “0.5C” was adopted as the upper limit value of the chord length c1 of the small blade 12, and “0.2C” as the lower limit value.

A description will be given of analysis on tip vortices carried out for setting an upper limit value and a lower limit value on the specific length b1 of the rotor blade 10 according to the embodiment, referring to FIGS. 5 and 6.

Relating to the embodiment, tip vortices generation states are analyzed in the case that a main blade of a rotor blade is provided with a small blade having the blade tip part protruding outward by the specific length b1 relative to the blade tip part of the main blade. Based on the analysis result, a range of the specific length b1 (upper limit value and lower limit value) is extracted so as to obtain an expected diffusion effect on tip vortices, and applied the specific length b1 to the small blade 12 of the rotor blade 10. Here, the main blade used in this analysis has a rectangular blade tip part.

FIG. 5A is a graph showing a tip vortices generation states in the case that the main blade only is used. FIGS. 5B and 5C are graphs showing tip vortices generation states in the case that the main blade is provided with a small blade having the blade tip part protruding outward by the specific length b1 relative to the main blade tip. The specific length b1 is set to 20% of the chord length C of the main blade in FIG. 5B, and 40% in FIG. 5C.

The ordinate and abscissa in FIG. 5 are set to the same values as in FIG. 3, respectively, and vorticity contour lines h and slip flow velocity vectors v at a downstream position with one chord length of the main blade from the main blade are shown as in FIG. 3. The broader vorticity contour lines h and the shorter slip flow velocity vectors v mean the better vortex diffusion.

In the case that the main blade only is used (without a small blade) as shown in FIG. 5A, the contour lines h are relatively dense and the velocity vectors v are relatively longer, therefore it is understood that tip vortices shed from the blade are relatively strong. Thus, if the small blade is not provided, the tip vortices shed from the main blade are not diffused and weakened. Even if a small blade is provided, when the specific length b1 is almost zero with a short span length of the small blade, a vortex center position of the tip vortices shed from the blade tip part of the small blade is close to that shed from the blade tip part of the main blade, therefore these two vortices can not be effectively interfered with each other.

In the case that the specific length b1 is set to 20% and 40% of the chord length C of the main blade as shown in FIGS. 5B and 5C, on the contrary, the contour lines h become relatively broader and the slip flow velocity vectors v become relatively shorter, hence it is understood that tip vortices shed from the blade are relatively weaker. The reason for this is: if the specific length b1 is set to a proper value, the vortex center position of the tip vortices shed from the blade tip part of the main blade is separated apart from that shed from the blade tip part of the small blade by a proper distance in the span direction, therefore these two vortices can be effectively interfered with each other.

Incidentally, if the specific length b1 is set to more than 50% of the chord length C, the intensity of joint between the small blade and the main blade becomes unfavorably lower.

In a graph of FIG. 6, the ordinate denotes the maximum swirl velocity of tip vortices shed from the rotor blade, and the abscissa denotes values that the specific length b1 is divided by the chord length C of the main blade. As shown by a dotted line in FIG. 6, if the main blade is not provided with a small blade, the maximum swirl velocity of tip vortices presents a constant high value. It is understood, on the contrary, that, if the main blade is provided with the small blade having the blade tip part protruding outward by the specific length b1 (for example, 20% to 40% of the chord length C of the main blade) relative to the blade tip part of the main blade, the maximum swirl velocity of tip vortices is greatly reduced due to vortex diffusion of the small blade tip part.

Based on the analysis result described above relating to the rotor blade 10 according to the embodiment, “0.4C” was adopted as the upper limit value of the specific length b1, and “0.2C” as the lower limit value.

Referring to FIG. 7, a description will be given of a noise reduction effect confirmed by flight tests using an actual helicopter.

FIG. 7 shows a measured result of landing noise for the case that the rotor blade has a main blade only and that for the case that the rotor blade has a main blade with a small blade. Measured landing noise mostly comes from BVI noise. In this test, the chord length c1 of the small blade is set to 30% of the chord length C of the main blade. The blade tip of the small blade protrudes outward by the specific length b1 relative to the blade tip part of the main blade, and the specific length b1 is set to 30% of the chord length C.

As shown in FIG. 7, the landing noise (BVI noise) for the case that the rotor blade has the main blade with the small blade is reduced by “2.8 EPNdB” compared with that for the case that the rotor blade has the main blade only. It is proved that, by setting the chord length of the small blade to a proper value and protruding the blade tip of the small blade by the specific length relative to the blade tip part of the main blade with the specific length set to a proper value, the tip vortices shed from the blade tip part of the main blade interfere with the tip vortices shed from the blade tip part of the small blade, which weakens the tip vortices and resultantly reduces the BVI noise.

In the rotor blade 10 according to the embodiment described above, since the small blade 12 is attached and extended along the straight line part of the leading edge 11 a of the main blade 11, tip vortices can be shed from both the tip part of the main blade 11 and the blade tip part of the small blade 12.

Since the chord length c1 is set to a certain length (20% or more and 50% or less of the chord length C of the main blade 11), the intensity of the tip vortices (swirl velocity) shed from the blade tip part of the main blade 11 and that shed from the blade tip part of the small blade 12 can be properly set. Further, since the blade tip of the small blade 12 protrudes outward by the specific length b1 (20% or more and 40% or less of the chord length C of the main blade 11) relative to the tip part of the main blade 11, the vortex center position of the tip vortices shed from the blade tip part of the main blade 11 can be separated apart from that shed from the blade tip part of the small blade 12 by a proper distance in the span direction.

Accordingly, the tip vortices shed from the tip part of the main blade 11 and that shed from the blade tip part of the small blade 12 can be positively interfered with each other and diffused. As a result, the tip vortices shed from the blade tip part of the rotor blade 10 can be weakened, which allows remarkable reduction of BVI noise when the helicopter lands.

Moreover, the small blade 12.of the rotor blade 10 according to the embodiment described above has a simple plane shape (rectangular shape), therefore the time and cost required for designing and manufacturing the small blade 12 can be greatly reduced. Further, since the rotor blade 10 according to the invention can be manufactured by modifying a conventional rotor blade at its blade tip part only, it is possible to reduce the time and cost required for manufacturing a whole rotor blade.

Furthermore, the rotor blade 10 according to this embodiment has a blade tip end 13 of the main blade 11 formed in a parabolic shape, which makes airspeed in a direction perpendicular to the blade tip end 13 of the main blade 11 smaller toward the leading edge side. Accordingly, the rotor blade 10 according to the embodiment is superior in a transonic characteristic and delays the generation of a shock wave, thereby avoiding a sharp increase of resistance.

Second Embodiment

A description will be given of a rotor blade 10A according to a second embodiment of the present invention with reference to FIG. 8. The rotor blade 10A is substantially the same as that of the first embodiment with the exception that the blade tip vicinity part of the main blade and the construction of the small blade of the rotor blade 10 in the first embodiment are slightly modified. Therefore, those elements which are the same as corresponding elements in the first embodiment are designated by the same reference numerals as of the first embodiment, and the modified structure only will be described.

As shown in FIG. 8, the rotor blade 10A according to this embodiment has a blade tip vicinity part 11A of the main blade 11 bent downward by a predetermined anhedral angle δ1, and has a small blade 12A attached to the main blade 11 by a predetermined anhedral angle δ2 with respect to the main blade 11. These anhedral angles δ1 and δ2 can be properly determined according to the size and rotating velocity of the rotor blade 10A, the flying speed of the helicopter, etc., and can be set to, for example, within a range of 0-30 degrees.

In this embodiment, the anhedral angle δ1 of the blade tip vicinity part 11A of the main blade 11 is set to larger angle than the anhedral angle δ2 of the small blade 12A as shown in FIG. 8B. With this structure, tip vortices generated by the blade tip part of the main blade 11 is positioned under tip vortices generated by the blade tip part of the small blade 12A, and these two tip vortices can be effectively interfered with each other. Further, since the tip vortices are shed downward from the rotor blade 10A, these vortices are hard to interact with the succeeding rotor blade, which can reduce BVI noise all the more.

The rotor blade 10A of the embodiment can shed tip vortices downward, and therefore can control the partial stall of the succeeding rotor, the stall generally being caused by the current induced by the tip vortices. Resultantly, there can be reduced energy loss during the drive of a rotary wing, and be improved hovering performance while the helicopter is suspended in the air.

Third Embodiment

A description will be given of a rotor blade 10B according to a third embodiment of the present invention with reference to FIG. 9. The rotor blade 10B is substantially the same as that of the first embodiment with the exception that the construction of the small blade of the rotor blade 10 in the first embodiment is slightly modified. Therefore, those elements which are the same as corresponding elements in the first embodiment are designated by the same reference numerals as of the first embodiment, and the modified structure only will be described.

As shown in FIG. 9, the rotor blade 10B has a small blade 12B attached to the main blade 11 by a predetermined angle of incidence θ. The angle of incidence θ can be properly determined according to the size and rotating velocity of the rotor blade 10A, the flying speed of the helicopter, etc., and can be set to, for example, within a range of −5 to +5 degrees. By setting the angle of incidence θ to a proper value, tip vortices shed from the blade tip part of the small blade 12B can be adjusted in the flowing direction and intensity thereof so that the tip vortices shed from the blade tip part of the main blade 11 can effectively interfere with the tip vortices shed from the blade tip part of the small blade 12B.

Fourth Embodiment

A description will be given of a rotor blade 10C according to a fourth embodiment of the present invention with reference to FIG. 10. The rotor blade 10C is substantially the same as that of the first embodiment with the exception that the construction of the joined potion between the main blade and the small blade of the rotor blade 10 in the first embodiment is slightly modified. Therefore, those elements which are the same as corresponding elements in the first embodiment are designated by the same reference numerals as of the first embodiment, and the modified structure only will be described.

As shown in FIG. 10, the rotor blade 10C has a joined potion 14C formed of a smooth surface (plane or curved surface) between the main blade 11 and the small blade 12. The joined portion 14C having a smooth surface can reduce air resistance acting thereon, which can improve a transonic characteristic and delay the generation of a shock wave.

Fifth Embodiment

A description will be given of a rotor blade 10D according to a fifth embodiment of the present invention with reference to FIG. 11. The rotor blade 10D is substantially the same as that of the first embodiment with the exception that the construction of the blade tip vicinity part of the main blade of the rotor blade 10 in the first embodiment is slightly modified. Therefore, those elements which are the same as corresponding elements in the first embodiment are designated by the same reference numerals as of the first embodiment, and the modified structure only will be described.

As shown in FIG. 11, the rotor blade 10D has a straight blade tip end 13D connecting between tips of the leading edge 11 a and the trailing edge 11 b of the main blade 11. In the rotor blade 10D of this embodiment, the trailing edge 11 b protrudes outward relative to the leading edge 11 a, then the straight blade tip end 13D has a large swept-back angle Λ, thereby airspeed perpendicular to the blade tip end 13D is reduced due to the effect of the swept-back angle Λ. That is, if a constant flow velocity is given by V∞, the airspeed perpendicular to the blade tip end 13D is V∞cosΛ (<V∞). Accordingly, the rotor blade 10D according to this embodiment is superior in a transonic characteristic and delays the generation of a shock wave, allowing avoidance of a sharp increase of resistance.

Sixth Embodiment

A description will be given of a rotor blade 10E according to a sixth embodiment of the present invention with reference to FIG. 12. The rotor blade 10E is substantially the same as that of the first embodiment with the exception that the construction of the blade tip vicinity part of the main blade of the rotor blade 10 in the first embodiment is slightly modified. Therefore, those elements which are the same as corresponding elements in the first embodiment are designated by the same reference numerals as of the first embodiment, and the modified structure only will be described.

As shown in FIG. 12, in the rotor blade 10E according to this embodiment, a leading edge blade tip vicinity part 15E is swept back by a certain swept-back angle, a trailing edge blade tip vicinity part 16E is swept back by the certain swept-back angle to be in parallel with the leading edge blade tip vicinity part 15E, the trailing edge blade tip vicinity part 16E is extended outward, and the tip of the leading edge blade tip vicinity part 15E is connected with the tip of the trailing edge blade tip vicinity part 16E with a blade tip end 13E having a smooth parabolic shape. The small blade 12 has a leading edge 12 a continuous with the leading edge blade tip vicinity part 15E. That is, the rotor blade 10E according to this embodiment has a blade tip vicinity part 11E of the main blade 11 and the small blade 12 continuously joined to the blade tip vicinity part 11E so as to sweep back toward the blade tip outside.

In the rotor blade 10E according to this embodiment, the structure is such that the blade tip vicinity part 11E of the main blade 11 and the small blade 12 continuously joined to the blade tip vicinity part 11E sweep back toward the blade tip outside. Thus, airspeed in the direction perpendicular to the blade tip vicinity part 11E and the small blade 12 of the main blade 11 can be reduced. Consequently, the rotor blade 10E according to this embodiment is superior in a transonic characteristic and delays the generation of a shock wave, allowing avoidance of a sharp increase of resistance.

In the embodiments described above, the trailing edge of the main blade of the rotor blade exemplarily protrudes outward relative to the leading edge, but it is not always necessary to protrude the trailing edge outward than the leading edge. A main blade having a rectangular blade tip part can be employed. Even when the main blade having a rectangular blade tip part is employed, aforementioned effects of the vortex diffusion and the noise reduction can be achieved, as long as the chord length c1 of the small blade is set to a particular length (for example, 0.2C≦c1≦0.5C), and the blade tip of the small blade protrudes outward by the specific length b1 relative to the blade tip part of the main blade, with the specific length b1 set to a certain length (for example, 0.2C≦b1≦0.4C).

In the embodiments described above, the chord length c1 of the small blade of the rotor blade is exemplarily set to 0.5C (50% of the main blade chord length C) as the upper limit value, and 0.2C (20% of the main blade chord length C) as the lower limit value. However, these upper and lower limit values can be appropriately changed according to the size and shape of a rotor blade, if desired tip vortex diffusion could be attained. In this case, the lower limit value of the chord length c1 of a small blade should be set to not less than a minimum necessitated value α (>0) to function as a small blade.

In the embodiments described above, the specific length b1 of the rotor blade is exemplarily set to 0.4C (40% of the main blade chord length C) as the upper limit value, and 0.2C (20% of the main blade chord length C) as the lower limit value. However, these upper and lower limit values can be appropriately changed according to the size and shape of a rotor blade, if desired tip vortex diffusion could be attained. For example, the upper limit value of the specific length b1 can be set to 0.5C (50% of the main blade chord length C), and the lower limit value to a positive constant value β (>0) less than 0.2C.

In the second embodiment, the anhedral angle δ1 of the blade tip vicinity part 11A of the main blade 11 is exemplarily set to larger than the anhedral angle δ2 of the small blade 12A (δ1>δ2), but the anhedral angle δ2 may be set to larger than the anhedral angle δ1 (δ2>δ1). In this case, the tip vortices shed from the blade tip part of the small blade 12A can be positioned under that shed from the blade tip part of the main blade 11, and these two tip vortices can be effectively interfered with each other.

While there has been described in connection with the preferred embodiments of the present invention, it is to be understood to those skilled in the art that various changes and modifications may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the present invention. 

1. A rotor blade for a helicopter comprising: a main blade with a blade root attached to a rotor shaft of the helicopter; and a small blade attached to a blade tip part of the main blade, wherein a tip part of the small blade is rectangular form in its plane shape, and has a leading edge continuous with a leading edge of the main blade and a chord length shorter than a chord length of the main blade.
 2. The rotor blade for a helicopter as claimed in claim 1, wherein the chord length c1 of the small blade is set so as to satisfy a following relational expression; α≦c1≦0.5C (α>0) where C denotes the chord length of the main blade.
 3. The rotor blade for a helicopter as claimed in claim 1, wherein the chord length c1 of the small blade is set so as to satisfy a following relational expression; 0.2C≦c1≦0.5C where C denotes the chord length of the main blade.
 4. The rotor blade for a helicopter as claimed in claim 1, wherein a blade tip of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is so set as to satisfy a following expression; β≦b1≦0.5C (β>0) where C denotes the chord length of the main blade.
 5. The rotor blade for a helicopter as claimed in claim 1, wherein a blade tip of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is set so as to satisfy a following expression; 0.2C≦b1≦0.4C where C denotes the chord length of the main blade.
 6. The rotor blade for a helicopter as claimed in claim 1, wherein a blade tip vicinity part of the main blade is bent downward by a predetermined anhedral angle.
 7. The rotor blade for a helicopter as claimed in claim 1, wherein the small blade has a predetermined anhedral angle relative to the main blade.
 8. The rotor blade for a helicopter as claimed in claim 1, wherein the small blade has a predetermined angle of incidence relative to the main blade.
 9. The rotor blade for a helicopter as claimed in claim 1, wherein a blade tip vicinity part of the main blade and the small blade continuously joined to the blade tip vicinity part are swept back toward the outside of the blade tip.
 10. The rotor blade for a helicopter as claimed in claim 2, wherein a blade tip part of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is set so as to satisfy a following expression; β≦b1≦0.5C (β>0) where C denotes the chord length of the main blade.
 11. The rotor blade for a helicopter as claimed in claim 3, wherein a blade tip of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is set so as to satisfy a following expression; β≦b1≦0.5C (β>0) where C denotes the chord length of the main blade.
 12. The rotor blade for a helicopter as claimed in claim 2, wherein the blade tip of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is set so as to satisfy a following expression; 0.2C≦b1≦0.4C where C denotes the chord length of the main blade.
 13. The rotor blade for a helicopter as claimed in claim 3, wherein the blade tip of the small blade protrudes outward by a specific length relative to a tip part of the main blade, and the specific length b1 is set so as to satisfy a following expression; 0.2C≦b1≦0.4C where C denotes the chord length of the main blade.
 14. The rotor blade for a helicopter as claimed in claim 8, wherein the angle of incidence is set to be in a range of −5 to +5 degrees. 